A flat panel display device (FPD) such as an organic EL (Electro-Luminescence) display (OELD), a plasma display panel (PDP) or a liquid crystal display device (LCD) has such a structure that a glass substrate for an element having a display element such as a light-emitting element formed and a glass substrate for sealing are disposed to face each other and the light-emitting element is sealed in a glass package comprising two such glass substrates that are sealed together (Patent Document 1). Also for a solar cell such as a dye-sensitized solar cell, application of a glass package having a solar cell element (photoelectronic conversion element) sealed with two glass substrates have been studied (Patent Document 2).
As a sealing material to seal a space between two glass substrates, application of sealing glass excellent in the moisture resistance, etc. is in progress. Since the sealing temperature of the sealing glass is at a level of from 400 to 600° C., properties of an electronic element portion of the OEL element or the dye-sensitized solar cell will be deteriorated when firing is conducted by using a conventional heating furnace. Accordingly, it has been attempted that a sealing material layer (a layer of a glass material for sealing) containing a laser absorbent is disposed between sealing regions provided on the peripheral portions of two glass substrates, and the layer is irradiated with a laser light to heat and melt the layer thereby to form a sealing layer (Patent Documents 1 and 2).
Sealing by laser irradiation (laser sealing) can suppress thermal influences over the electronic element portion, and on the contrary, it has a disadvantage that cracks, fractures and the like are likely to form on the glass substrate. One reason of the problem may be the difference in the thermal expansion coefficient between the glass substrate and the sealing glass. With respect to this point, Patent Document 2 describes a sealing material having a thermal expansion coefficient within 10×10−7/° C. from that of the glass substrate. Since the thermal expansion coefficient of a sealing glass is larger than that of a glass substrate in general, besides a laser absorbent, a low-expansion filler such as silica, alumina, zirconia or cordierite is added to the sealing glass to obtain a sealing material having a low expansion coefficient.
By the way, reduction of the thickness of a glass package constituting e.g. a FPD or a solar cell tends to be in progress, and for this purpose, it is required to narrow the gap between the glass substrates to be, for example, at most 15 μm. Since e.g. the low-expansion filler is added to the sealing material as described above, it becomes necessary to reduce the particle size of the filler particles along with narrowing of the substrate distance. Reduction of the particle size of the filler particles causes increase of the specific surface area, and a sharing stress between the sealing glass melted by heat of the laser light and the filler particles increases to reduce fluidity. Accordingly, it becomes necessary to increase the process temperature (heating temperature) by the laser light, but when the process temperature is increased, a problem such that a crack or breakage etc. tends to occur in the glass substrates or the sealing layer.